The present disclosure relates to methods for making thermoplastic elastomer foams and foamed articles.
This section provides background information related to this disclosure but which may or may not be prior art.
Polyurethane foams are typically prepared by using chemically-acting blowing agents or physically-acting blowing agents that are mixed into or injected into the monomer reactants during polymerization. As an example, chemical blowing agents like water or formic acid form gaseous products by reaction with isocyanate groups, while physical blowing agents are dissolved or emulsified in the monomers and vaporize under polyurethane polymerization conditions. Other polymer foams may be made using azo compounds, hydrazine, or sodium bicarbonate. Examples of physical blowing agents include hydrocarbons, halogenated hydrocarbons, and carbon dioxide. Physical blowing agents are typically introduced either in-line, i.e. directly into the mixing head, or via a stock tank in a batch operation. Such a process is described, for instance, in Bruchmann et al., US Patent Application Publication No. US 2011/0275732.
Many physical properties of foams depend in large part on the cell morphology of the foam, including compressive strength, thermal conductivity, dimensional stability, and water absorption rate. However, it is difficult to control polymer foaming to the degree necessary for consistent production of a cell morphology that will produce a particular foam property, like good compressive strength, when making uncrosslinked (thermoplastic) foams. Prior art attempts to make foam micro-structures having desirable cell morphologies have included the use of powdered nucleation agents. Among these nucleating agents, inorganic oxides, such as talc, titanium dioxide, and kaolin have been used. Nucleation efficiency and, consequently, cell size and shape depend on the nucleating agent's particle size, shape, and surface treatment and distribution in the material being foamed. However, adding these nucleating agents can adversely affect other foam properties.
A need remains for improved methods of forming polyurethane foams, especially thermoplastic polyurethane foamed, that provide a cell structure for improved properties.